This invention relates to thermoformable polyurethanes prepared by the reaction of aliphatic or cycloaliphatic polyisocyanates with polyester, polycarbonate, and/or polyester carbonate diols having a molecular weight of from 2000 to 5000 and diol chain extenders having an average molecular weight of from 90 to 530. The polyurethanes of the invention are hard and flexible at room temperature but soften and become formable with moderate heating to temperatures below 100.degree. C.
Thermoformable plastics that soften at moderate temperatures but reharden upon cooling can be used as orthopedic splints and casts. For example, U.S. Pat. No. 4,784,123 discloses a thermoplastic polymeric material that can be formed at moderate temperatures comprised of a blend of an aliphatic polyester having a crystalline melting point of 50.degree. to 70.degree. C. and a thermoplastic polyurethane. The patent does not, however, disclose polyurethanes that have the desired properties when used alone.
Polyurethanes that can be thermally deformed at temperatures below 100.degree. C. are known. For example, U.S. Pat. No. 4,473,671 discloses thermoformable compositions prepared by the reaction of certain diisocyanates, isocyanate-reactive polyols or polyamines, and fillers. In particular, the patent describes compositions based on aromatic diisocyanates, polyethers or polyesters, optional chain extenders (although no examples include chain extender), and fillers. The fillers are described as essential for obtaining the desired combination of properties. When aliphatic or cycloaliphatic polyisocyanates are substituted for aromatic polyisocyanates according to the examples, the product is brittle. The addition of a chain extender would normally be expected to increase hardness and brittleness in polyurethanes.
It has now surprisingly been found that the reaction products of aliphatic or cycloaliphatic polyisocyanates, high molecular weight polyester, polycarbonate, and/or polyester carbonate diols having average molecular weights of from 2000 to 5000, and diol chain extenders having average molecular weights of from 90 to 530 are hard, yet flexible at room temperature. When prepared in the absence of chain extenders, polyurethanes based on the same polyisocyanates and polyesters and/or polycarbonates are brittle at ambient temperature, as reflected by their poor elongation properties. The addition of the specified amounts of chain extenders according to the invention results in a significant increase in elongation properties. The resulting polyurethanes are flexible and no longer brittle at ambient temperature.
The polyurethanes of the invention are also opaque at room temperature, even when opaque fillers are absent. Upon moderate heating, polyurethanes according to the invention soften and become formable and, if untilled, also become optically clear. Upon cooling, these polyurethanes regain their hardness, become unformable, and again become opaque, typically in less than about 15 minutes.